Photo-electric tube



June 1934- J. H. DE BOER El AL 1,952,895

PHOTO ELECTRIC TUBE Filed Sept. 13. 1929 INVENTOR$ JMl HENDRIK DE BOER AND BY mama cox us raves Anoiiu Patented June 12, 1934 1 UNITED STATES laws? PATENT orrica Eindhoven,

Netheriands, assignors to Radio Corporation of America, a corporation of Dela- V] are Application September 13, 1929, Serial No. 392,306 In the Netherlands September 25, 19 28 7 Claims.

Photo-electric tubes may be used for control' ling electric currents by light and are often taken advantage of, for example in installations for transmitting photos by Wireless telegraphy or along electric conductors and in television-apparatus in order to convert light variations into corresponding current variations. Such a photoelectric tube contains a photo-electric substance which on radiation emits electrons, the number which depends on the intensity of the radiation. If in the tube an electrode is provided which is connected by a source of voltage to the photo-electric substance, care being taken that the electrode obtains a positive potential with re spect to the photoelectric substance, the electrons emitted by this subsance will bring about a current through the tube and the circuit connected thereto, the strength of said current depending on the intensity of the radiation of the photo-electric substance. As a rule the current passing through the tube is very low and mostly has to be amplified for practical use.

The invention has for its object to provide a photo-electric tube having a comparatively intense electronic emission.

In a photo-electric tube according to the invention a substratum and the photo-electric substance which preferably consists of an alkaline metal, for example caesium, have Stratified between them a material which contains a chemical compound. The latter is preferably such that it adsorbs the photo-electric substance better than the material of which the substratum is composed. The chemical compound may consist with advantage of an oxide or a halogenide, for example calcium fluoride. As an alternative the chemical compound may be formed by etching of the substratum. The latter is preferably applied by volatilization of the compound in a high vacuum.

The invention will be more clearly understood by reference to the accompanying drawing in which an embodiment of the invention is diagrammatically illustrated. In the said drawing:

Figure 1 shows a photo-electric tube according to the invention, and

Figure 2 is a detail View.

The photo-electric tube shown in the figure, which may contain a gaseous atmosphere, for example of neon or which may be exhausted to the highest possible extent has a sealed envelope with a wall 1 which may consist, for example, of glass and which has mounted on it a layer of caesimn 2. This substance has, however, not been applied directly to the wall of the tube but to an after the exhaustion of the tube is volatilized, for

which purpose a current is passed through the electrode by means of the leading-in wires 7 and 8. The calcium fluoride volatilized is deposited on V the wall of the tube where it forms the layer 3. The caesium may be introduced into the tube by volatilization from a side-tube which may be connected, for example, to 9.A suitable method of introducing the caesium into the tube is described, for example, in the United States application Serial No. 360,953, filed May 6, 1929, by Jan H. de Boer.

When operating the said tube the electrode or anode 6 is given a positive potential with respect to the photo-electric electrode or cathode and the latter is radiated by light which can enter the tube through the window 10 and which dislodges electrons from the photo-electric layer, the number of liberated electrons depending on the intensity of the exposure. The current passing through the tube is consequently a measure of the exposure of the photo-electric electrode or cathode and variations in the exposure will result in corresponding variations in the current.

The current working of the tube according to the invention may be due to the intermediate layer having a rough surface so that the photoelectric substance deposited thereon acquires a large area.

If in a photo-electric tube the photo-electric substance is applied directly to the glass wall, the layer applied is frequently no longer coherent. If on the contrary as the intermediate layer in a tube according to the invention a compound is used which adsorbs the photo-electric substance better than glass, a very good coherent photo-electric layer can be produced. Consequently the vapour pressure of the photo-electric substance may be low. In the adsorbed state the photo-electric substance may have a colour different from that in the non-adsorbed state. Caesium applied, for example, to calcium fluoride as the intermediate layer is blue, if applied to sodium bromide it is green. Thus it is rendered possible to control the spectral sensitiveness of the tube by a suitable choice of the chemical compound as the intermediate layer.

What we claim is:

1. A photoelectric tube having a stratum of calcium fluoride on the inner surface of the tube wall, and an alkali metal coating forming a photo-electric cathode, and an anode cooperating therewith.

2. A photo electric tube comprising a sealed envelope, an anode, and a cathode comprising a stratum consisting essentially of a halogenide of an alkaline earth metal, and a film of photo sensitive metal adsorbed on the surface of said stratum.

3. A photo electric tube comprising a sealed envelope, an anode, and a cathode comprising a stratum consisting essentially of a halogenide of calcium, and a film of photo sensitive metal adsorbed on the surface of said stratum.

4. A photo electric tube comprising a sealed envelope, an anode, and a cathode comprising a stratum consisting essentially of a fluoride of an alkaline earth metal, and a film of photo sensitive metal adsorbed on the surface of said straturn.

5. A photo electric tube comprising a sealed envelope, an anode, and a cathode comprising a stratum consisting essentially of calcium fluoride, and a film of caesium adsorbed on the surface of said stratum. I

6. A photo electric tube comprising a sealed envelope, an anode, and a cathode comprising a stratum consisting essentially of a thin layer of calcium fluoride on the inner surface of said envelope, and a film of photo sensitive metal on said layer.

7. A photo electric tube comprising a sealed envelope, an anode, and a cathode comprising a stratum consisting essentially of a thin layer of non-oxidizing halogenide of an alkaline earth metal on the wall, and a film of adsorbed caesium on said layer.

JAN HENDRIK DE BOER. MARTEN CORNELIS TEVES. 

